Field of the Invention
The present invention relates to an image processing apparatus and an image processing method used in ophthalmic practices.
Description of the Related Art
Subjects' eyes are often examined for the purpose of early diagnosis and treatment of lifestyle-related diseases and diseases that are leading causes of blindness. A scanning laser ophthalmoscope (SLO), which is an ophthalmic apparatus using the principles of confocal laser microscopes, is configured to raster-scan a laser beam (measuring beam) across a fundus and quickly acquire a high-resolution planar image from the intensity of light returned from the fundus.
By detecting only light that has passed through an opening (pinhole), only returned light at a particular depth can be converted into an image, which has a higher contrast than images that can be acquired by a fundus camera.
Hereinafter, an apparatus configured to capture such a planar image will be referred to as an SLO apparatus, and the planar image will be referred to as an SLO image.
By increasing the diameter of a measuring beam in the SLO apparatus, it has become possible in recent years to acquire an SLO image of a retina with higher lateral resolution. However, as the diameter of the measuring beam increases, degradation in signal-to-noise (S/N) ratio and resolution of the SLO image caused by aberration of the subject's eye has become a problem in acquiring an SLO image of the retina.
As a solution to this, an adaptive optics SLO apparatus including an adaptive optics system has been developed. The adaptive optics system is configured to measure the aberration of the subject's eye with a wavefront sensor in real time, and correct, with a wavefront correction device, the aberration of the measuring beam or returned light occurring in the subject's eye. With this adaptive optics SLO apparatus, an SLO image with high lateral resolution can be acquired.
This SLO image with high lateral resolution can be acquired as a moving image. For example, to noninvasively observe the circulation of blood, retinal vessels are extracted from each frame, and the transfer rate of blood cells in capillaries is measured. Also, to evaluate a relation with a visual function using the SLO image, visual cells P are detected, and the density distribution and arrangement of the visual cells P are measured. FIG. 6B illustrates an SLO image with high lateral resolution. This SLO image allows observation of a low-luminance region Q corresponding to the position of the visual cells P and capillaries, and a high-luminance region W corresponding to the position of a white blood cell.
For observation of the visual cells P, an SLO image, such as that illustrated in FIG. 6B, is captured with the focus position set near the outer layer of the retina (B4 in FIG. 6A). Retinal vessels and branched capillaries run through the inner layers of the retina (B1 to B3 in FIG. 6A). When an adaptive optics SLO image is acquired with the focus position set in an inner layer of the retina, it is possible to directly observe retinal vascular walls.
However, in a confocal image of the inner layer of the retina, strong noise signals caused by reflection of light from a nerve fiber layer may make it difficult to observe a vascular wall and detect wall boundaries.
Accordingly, a method of observing a non-confocal image has begun to be used in recent years. The non-confocal image is obtained by acquiring scattered light by varying the diameter, shape, and position of a pinhole in front of a light receiving portion. A large focus depth of the non-confocal image facilitates observation of an object having protrusions and recesses in the depth direction, such as a vessel. Also, since light reflected from the nerve fiber layer is not easily directly received, it is possible to achieve noise reduction.
A retinal artery is a small artery (arteriole) with a vessel diameter of about 10 μm to 100 μm. The wall of the retinal artery includes an intima, a media, and an adventitia. The media is formed by smooth-muscle cells, and runs in a coil-like manner in the circumferential direction of the vessel. For example, if hypertension causes increased pressure on the retinal arterial wall, the smooth muscle contracts and the wall thickness increases. At this point, the shape of the retinal arterial wall can be restored when the blood pressure is lowered, for example, by taking a blood pressure lowering drug. However, if the hypertension is left untreated for a long time, the smooth-muscle cells forming the media become necrotic, and fibrous thickening of the media and adventitia leads to an increase in wall thickness. At this point, organic (irreversible) damage already develops in the retinal arterial wall, and continuous treatment is required to prevent worsening of the arteriolar damage.
A technique for measuring retinal vessel diameters is disclosed in “Computer algorithms for the automated measurement of retinal arteriolar diameters” by Chapman et al., published in Br J Ophthalmol, Vol. 85, No. 1, pp. 74 to 79, 2001. In this technique, a luminance profile generated on a line segment substantially perpendicular to the running of a retinal vessel in an SLO image is linearly approximated for each small window. Then, positions corresponding to the maximum and minimum values of the slope of the resulting regression line are acquired as retinal vessel boundaries to measure the retinal vessel diameter. Additionally, a technique for semi-automatically extracting retinal vascular wall boundaries in an adaptive optics fundus camera image using a variable geometry model is disclosed in “Morphometric analysis of small arteries in the human retina using adaptive optics imaging: relationship with blood pressure and focal vascular changes” by Koch et al., published in Journal of Hypertension, Vol. 32, No. 4, pp. 890 to 898, 2014.